How much binder are we truly getting from RAP in today’s mixtures? And how relevant is that contribution in modern mix design?

As agencies continue to adopt higher RAP contents, questions remain about the contribution of aged binder from RAP. RAP contains binder that is commonly assumed to partially blend with virgin binder, and concepts such as Recycled Binder Availability (RBA) are used to estimate how much of this binder is effectively active in the mixture. As RAP usage increases and performance-based mix design approaches are adopted, how important is it to understand and define RAP binder contribution in mixture design and evaluation?

Josue Garita-Jimenez, MSc. NCAT Ph.D. Student

How much recycled binder actually becomes active in the mixture is a question that has a bigger impact than it may appear. As agencies push toward higher RAP and RAS contents driven by economic and environmental benefits, the answer directly shapes stiffness, cracking resistance, and long‑term durability.

Recycled Binder Availability (RBA) was developed to move beyond fixed blending assumptions and quantify what fraction of recycled binder is actually active in the mixture. The risk of getting RBA wrong cuts both ways. If the designer assumes RAM contributes more binder than it really does, the mixture ends up short on effective binder, leading to cracking and durability issues. If the designer assumes RAM contributes less than it really does, they compensate by adding more virgin binder than needed, and that excess binder increases rutting susceptibility. At low RAM percentages, the impact of this error is small. At 30, 40, or 50 percent RAM, the same error can move the mixture well outside its intended performance range.

Beyond how much recycled binder becomes active, the nature of that activated binder also matters. Aged binder is stiffer and has lower relaxation capacity than virgin binder, so a mixture with more activated recycled binder is not automatically a better‑performing mixture. More binder, yes, but also more stiffness, and stiffness without adequate relaxation is what drives cracking.

RBA plays a central role in volumetric mix design because the calculated virgin binder content depends directly on how much recycled binder is assumed to be active. Findings from NCHRP Project 09‑68 show that RBA is not an intrinsic property of the RAM. It is mixture‑specific, depending on RAM source, content, and type, virgin binder grade and source, rejuvenators, and production temperature. Assuming a constant RBA is therefore not defensible.

Used well, RBA becomes a tool for compensation and optimization. Designers can adjust virgin binder content to offset the inactive recycled fraction, producing mixtures that are more balanced and cost‑effective. As RAP and RAS contents continue to rise, accurately defining recycled binder contribution is essential for achieving the performance agencies expect.

Kazuo Kuchiisi, Ph.D. Research Scientist, Virginia Transportation Research Council (VTRC).

During mix design, RAP binder is often assumed to fully mobilize and blend with virgin binder. We all know that it’s not 100%. In fact, during production, part of the RAP acts as a ‘black rock’ and never sees the light—it doesn’t even see the virgin binder to begin with! As the industry continues moving towards higher RAP contents, properly accounting for the inactive RAP binder is essential to ensure asphalt mixtures are not too dry and crack susceptible. Yet uncertainty still exists on how much of the RAP binder actually blends with virgin binder.

With ongoing efforts to implement balanced mix design (BMD) across states, one may ask: doesn’t BMD address this uncertainty? Maybe, but we’re not there yet. Most agencies adopting or considering BMD still require volumetric checks as part of mix design. A performance only BMD approach could address RAP binder availability (RBA), but developing and validating such frameworks takes time. Additionally, not all mixture types currently have established BMD protocols. Different performance tests and criteria may be needed depending on binder type, gradation, layer within the pavement structure, and use of additives or modifiers. In the meantime, leveraging the RBA concept can offer some help.

Various methods exist to quantify RBA, varying in terms of cost, accuracy, and practicality. In Virginia, in particular, ongoing efforts to benchmark binder availability of RAP materials across the state are being conducted by leveraging simple and practical methods that could potentially be used by contractors in a daily basis. However, important questions remain: What mix design or production factors influence RBA? Does RBA truly improve asphalt pavements longevity? How should RBA be incorporated into quality control and quality assurance practices?

Will RBA solve all the mix performance issues? Definitely not. But the appropriate consideration of RBA can help verify and/or leverage benefits associated with BMD, especially for asphalt mixtures that do not have a BMD framework yet developed or that include RAP materials with a very low binder contribution.

RBA may not be the silver bullet, but it may serve as the compass that points us towards the right direction.

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Fan Yin, Ph.D., P.E. Assistant Director & Associate Research Professor Research | NCAT

The short answer to the first question is, “it depends.” The more important question, however, is what it depends on. In NCHRP Project 09 68, we found that the RBA of RAP mixtures depends not only on the properties of the RAP itself, but also on key mix design variables and production conditions. This finding suggests that RBA is an intrinsic property of the asphalt mixture as a whole, rather than a fixed property of the RAP material alone. In our study, RBA values for more than 20 RAP/RAS mixtures were found to range widely from 48% to 100%.

The importance of RBA in modern asphalt mix design also depends, specifically, on the mix design methodology being used. Under traditional volumetric mix design, RBA can be a powerful tool for improving the cracking resistance and durability of high RAP mixtures, because it accounts for the need to add additional virgin binder to compensate for partially inactive RAP binder.

In contrast, under a balanced mix design (BMD) framework, particularly a ‘true’ BMD approach that places minimal emphasis on constituent and volumetric requirements, the significance of RBA is greatly reduced. In this case, the optimum binder content is determined directly through performance testing of the final mixture, making the explicit determination of RBA unnecessary. Ultimately, performance based BMD testing allows mixture performance to be evaluated directly, regardless of the level of binder availability in the RAP.

R. Grover Allen Ph.D., P.E. Southeastern Regional Engineer, Asphalt Institute.

This is an excellent question! RBA is a concept that is widely unknown and even often misinterpreted in our industry. The main points I’d make about RBA and how important it is in today’s mix designs are as follows:

RAP binder is entirely inactive when sitting in a stockpile (unheated). We often refer to RAP with inactive binder as “black rock.” Heating asphalt, including RAP asphalt, to a high temperature (~325-350 °F) introduces a process that is supposed to break down weaker pi-pi bonds and allow the asphalt to temporarily flow and coat uncoated particles. Of course, these weak bonds in asphalt eventually re-form when the heat is removed, allowing these particles to be glued together and effectively making them “black rocks” once again.

RBA is intended to determine the level (or percentage) of binder activation that occurs during the intermediate phase between the stockpile and being placed in-service, i.e. during heating and mixing.

Does the heating and mixing process actually convert the very stiff RAP binder from “black rock” to fully-activated binder? NCHRP 648 (2010) revealed that a PG 90.0-17.8 binder (which is stiffer than typical virgin binders but softer than typical RAP binders) would typically be less than 90% activated; and further – that over 400 °F would be required to fully activate it (note: 350 °F is the maximum temperature mixtures can be heated to prevent damaging the binder).

RBA research by Georgia DOT, NCHRP 9-58, and other sources have further revealed that fully-activated RAP binder is a poor assumption in most cases, with 60-80% activation being the most common range adopted.

Bottom line: binder that doesn’t activate should be replaced to fulfill effective binder requirements. NCHRP 9-68 aims to further quantify RBA variables. The following articles provide a more in-depth discussion on this thought-provoking subject – RBA and how RBA relates to BMD.
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​https://www.asphaltmagazine.com/is-rap-just-a-black-rock/​

​https://www.asphaltmagazine.com/balanced-mix-design-recycled-binder-availability-and-how-they-fit/​

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